Metallic or metalloid fuels powders such as aluminum, boron, magnesium, silicon, lithium and alloys or combinations thereof have found use in various energetics including, for example, propellants, pyrotechnics and explosives. Aluminum has become one of the most frequently used metallic fuels in such energetics, yet its efficient use in these energetics remains challenging for several reasons. For example, in the use of micrometer sized aluminum in propellants, the relatively high ignition temperature of aluminum and related particle agglomeration typically results in lower combustion efficiency and increased two-phase flow losses, e.g., slag formation.
To overcome or combat these drawbacks, micrometer sized aluminum has been replaced with nanosized aluminum (nAl) in experimental propellants and has resulted in improved performance (e.g., shorter particle burning time, reduced metal agglomeration, decreased ignition delay, reduced condensed product size, and anticipated increases in propellant heat feedback).
For example, U.S. Pat. No. 7,524,355 and U.S. Patent Application Publication 2010/0032064 disclose nano composite energetic powders, such as composed of aluminum and a metal oxide oxidizer, prepared by what is termed “arrested reactive milling” and such as exemplified by high energy milling.
Unfortunately, the utility of nanosized aluminum is significantly reduced as such materials can exhibit a high oxide content and a high surface area (10-50 m2/g) that can lead to various processing issues.
Conventional aluminized solid propellant is a physical mixture of fuel (typically aluminum, boron, magnesium, silicon, or alloys thereof) particles and oxidizer (typically ammonium perchlorate (AP), ammonium nitrate, potassium perchlorate, etc.). These particles are combined in a cured rubber-based binder matrix, or other polymer composite. When the propellant burns, the solid surface composed of these materials regresses. The binder burns with the oxidizer, exposing metal particles, which once exposed, light and burn in the surrounding hot, oxygenated gas environment. Combustion of metal in this way is limited by the rate at which oxidizer gases can be diffused to the metal surface. As such, reaction rates can generally be improved by increasing metal-gas interface surface area. Furthermore, reaction of metal with oxidizer can create a partial metal oxide coating or “cap” on the surface of molten, burning metal, which further hinders the metal-oxygen reaction by forming a diffusive barrier at the surface of the burning particle. A second way in which metal combustion is hindered is related to the melting and agglomeration of conventional metal particles. Melting, which typically occurs at the propellant surface hinders reaction because molten metal particles tend to agglomerate together, reducing metal-oxidizer interfacial surface area. Furthermore, the time delay between when metal particles begin to melt and when they reach the ignition temperature provides molten particles ample time to coalesce and create larger agglomerates with lower specific surface area. These two problems, (1) the formation of a partial oxide layer on reacting particle surfaces and (2) the agglomeration of molten metal particles represent two significant deficiencies regarding metal combustion in a solid rocket motor.
Fluorocarbons are of particular interest for inclusion with aluminum and have been proposed in a variety of applications including reactive liners/fragments, heterogeneous explosives, and infrared (IR) flares. While fluorocarbons such as cause or result in the formation of metal fluorides are of interest, simple addition or coating is not effective. For example, coatings typically boil from the surface of reacting particles at temperatures below the melting point of metal oxides. Attempts have previously been made to introduce fluorine into a propellant. For example, U.S. Pat. No. 4,017,342 is directed to a method for improving the combustibility of aluminum metal powders for use in solid rocket propellant formulations and requires exposing aluminum oxide coated aluminum metal powder to hydrogen fluoride gas for a period of time sufficient to effect a reaction therebetween. Thus, the exterior surfaces of aluminum particles react with fluorine (from exposure of Al to HF). While in general, higher theoretical heat release and performance are possible from the formation of metal fluorides rather than metal oxides, such an aluminum fluoride coating on particle surfaces prior to combustion results in a lower overall heat release, as the aluminum particles contain an already reacted form of aluminum. U.S. Pat. Nos. 6,843,868 and 3,441,455 detail other attempts to introduce fluorine in the form of a fluorocarbon such as either physically mixed as a powder into the propellant prior to curing of the binder or as a coating, for example.
The success of metal-fluorocarbon reactives can predominantly be attributed to a very high (volumetric and gravimetric) heat release resulting from fluorination instead of oxidation. These benefits have been realized in reactive compositions where higher performance is seen from use of fluorine-based rather than oxygen based oxidizers. For applications where high gas production is desired (such as solid propellants), the about 1000° C. lower boiling/sublimation point of most metal fluorides compared to their respective oxides can decrease formation of condensed phase product. Reaction of Al with polytetrafluoroethylene (PTFE) is of particular interest due to PTFE's high fluorine content (67 mol. %) and the composition's high enthalpy of reaction (9 kJ/g).
However, one particular drawback of metal-PTFE reactives (as well as other heterogeneous reactives) is the large diffusion distances present in micron sized mixtures.
The issue of diffusion limited combustion has been addressed by several researchers either by significant reduction of reactant particle size through use of nanoparticle reactants (e.g., nAl-nPTFE) or mechanical activation (MA). The reduced diffusion distance resulting from the use of nanoscale particles dramatically decreases the thermal stimulus required to achieve ignition. Specifically, the heating of nAl-nPTFE (70-30 wt. %) mixtures has been shown to result in an exothermic pre-ignition reaction (PIR) at about 450° C., which is about 150° C. below the primary ignition temperature of micrometer scale Al-PTFE mixtures. In addition, the significantly higher heat release seen from nAl-nPTFE has been attributed to more complete combustion. The use of MA has been successfully applied to many heterogeneous energetics, as such processing provides a top-down approach to decreasing diffusion distances and altering ignition and reaction behavior. With MA (sometimes referred to as arrested reactive milling (ARM)), the milling process is interrupted prior to reaching a critical milling energy dose sufficient to induce self-sustained reaction. The milling yields increased reactant interfacial contact and decreased diffusion distances that can exceed that which is possible with nanoscale physical mixtures, which can lead to reaction at lower temperatures.
Also, the inclusion, by MA, of low levels (10 wt. %) of a secondary metal such as Fe, Zn, or Ni in aluminum has also been shown to reduce the ignition temperature and alter the low temperature oxidation process of aluminum. The addition of secondary metals in composite propellants, however, is not always advantageous and generally results in lower predicted specific impulse (Isp).
Thus, there remains a need and a demand for methods and materials such that can desirably facilitate the incorporation of metal fuels in various applications and uses.